what is the use of thunderbolt and can i transfer data between external hard disk and mac book air without using thunderbolt?

You can connect a monitor, a TV, or a device, such as an external storage device, to your Mac's Thunderbolt connector. You may also connect your Mac to a display that supports DisplayPort, Mini DisplayPort, HDMI, or VGA with the right adaptor.
A USB, USB-C, or Thunderbolt connection connects the two PCs. Do one of the following on the Mac you want to use as an external disc in target disc mode:
If the computer is turned off, hit and hold the T key to turn it on.
If the machine is turned on, go to Apple menu > System Preferences > Startup Disk > Target Disk Mode.
When the computer starts up, a disc icon displays on the other computer's desktop.
Drag and drop files to and from the hard drive.
Drag the disk's symbol to the Trash to eject it.
The Trash icon transforms into an Eject icon as you drag.
Disconnect the cord from the Mac you used as a disc by pressing the power button.

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